1. Field of the Invention
The invention is related to the field of electromagnetic induction well logging for determining the resistivity of earth formations penetrated by a wellbore. More specifically, the invention relates to measuring the transient signals in an induction tool having a metallic pipe with finite, non-zero and high conductivity.
2. Description of the Related Art
Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al. The instrument described in the Beard '761 patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages which are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
The development of deep-looking electromagnetic tools has a long history. Such tools are used to achieve a variety of different objectives. Deep looking tools attempt to measure the reservoir properties between wells at distances ranging from tens to hundreds of meters (ultra-deep scale). There are single-well and cross-well approaches, most of which are rooted in the technologies of radar/seismic wave propagation physics. This group of tools is naturally limited by, among other things, their applicability to only high resistivity formations and the power available down-hole.
At the ultra-deep scale, a technology may be employed based on transient field behavior. The transient electromagnetic field method is widely used in surface geophysics. Examples of transient technology are seen, for example, in Kaufman (1979) and Kaufman (1989). Sidorov et al., 1969, “Geophysical surveys with near zone transient EM.” Published by NVIGG, Saratov, Russia (in Russian); and Rabinovich et al., 1981, “Formation of an immersed vertical magnetic dipole field”: j. Geologiya I Geofizika, N 3.(in Russian). Typically, voltage or current pulses that are excited in a transmitter initiate the propagation of an electromagnetic signal in the earth formation. Electric currents diffuse outwards from the transmitter into the surrounding formation. At different times, information arrives at the measurement sensor from different investigation depths. Particularly, at a sufficiently late time, the transient electromagnetic field is sensitive only to remote formation zones and does not depend on the resistivity distribution in the vicinity of the transmitter (see Kaufman et al., 1983). This transient field is especially important for logging. A comprehensive study of deep-reading transient EM in wireline measurements has been conducted and discussed by Geldmacher et al., 1997 (“Single well (deep-reading) EM system.” Report on the pre-feasibility study, Western Atlas, Inc.). Use of a symmetric logging tool using transient field measurements for formation detection is discussed in U.S. Pat. No. 5,530,359, issued to Habasby et al.
U.S. Pat. No. 5,955,884, issued to Payton et al. discusses methods for measuring transient electromagnetic fields in rock formations. Electromagnetic energy is applied to the formation at selected frequencies and waveforms that maximize the radial depth of penetration of the magnetic and electric energy. Payton '884 comprises at least one electromagnetic transmitter and at least one electric transmitter for applying electric energy. The transmitter ands may be either single-axis or multi-axis electromagnetic and/or electric transmitter. In one embodiment the TEM transmitters and TEM receivers are separate modules that are spaced apart and interconnected by lengths of cable, with the TEM transmitter and TEM receiver modules being separated by an interval of one meter up to 200 meters as selected. Radial depth of investigation is related to the skin depth δ=√{square root over (2/σμω)} which in turn is related to frequency. Lower frequency signals can increase the skin depth. Similarly, the conductivity of the surrounding material inversely affects the skin depth. As conductivity increases the depth of investigation decreases. Finite conductivity casing of the apparatus therefore can reduce the depth of investigation.
Rapidly emerging measurement-while-drilling (MWD) technology introduces a new, meso-deep (3–10 meters) scale for an electromagnetic logging application related to well navigation in thick reservoirs. The major problem associated with the MWD environment is the introduction of a metal drill pipe close to the area being measured. This pipe produces a very strong response and significantly reduces the sensitivity of the measured EM field to the effects of formation resistivities and remote boundaries. The only previous solution for this problem has been to create a large spacing (up to 20 meters) between transmitter and receiver (as discussed in U.S. Pat. No. 6,188,222 B1, issued to Seydoux et al.). The sensitivity of such a tool to remote boundaries is low. Currently, Stolar Horizon, Inc. is developing drill string radar, DSR, for CBM (Coal Bed Methane) wells. DSR provides 3-D imaging within a close range of the wellbore.
There is a need for a method of processing data acquired with real MWD tools having finite non-zero conductivity in transient field studies. The present invention satisfies this need.